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The recent flood disaster in Utah that claimed at least 16 lives reminds us that except for heat-related fatalities more people die from flooding than any other meteorologically-caused phenomenon.
Flash floods are especially dangerous because they occur with little warning.
What exactly is a flash flood and what causes it?
Flood And Flash Flood
NOAA defines flood and flash flood as follows:
- Flood: An overflow of water onto normally dry land.
- Flash flood: A flood caused by heavy or excessive rainfall in a short period of time.
The key words here are “excessive rainfall” and “short period of time.”
The words excessive and short are vague, so let’s investigate what really goes into a flash flood. Clearly, for a flash flood to occur, the heavier the rainfall, the shorter the time needed to cause flooding.
In the recent flooding in Utah, the total amount of rain that fell would not normally be considered excessive.
Factors That Contribute To Flash Flooding: Soil Porosity
When rain falls on porous, unsaturated ground, it simply drains directly and rapidly. Various kinds of soil have different porosities, and rock has essentially none. In the case of the Utah flood, much of the rain fell on rock and immediately drained into the low-lying places.
Heavy rain can at first drain into porous soil, but eventually saturate it so that additional rain drains along the surface.
Factors That Contribute To Flash Flooding: Surface Drainage
When rain cannot drain directly into the ground, it seeks the easiest way out. Gutters and sidewalk drains are designed to direct water to where it can do no harm. Very heavy rain can overwhelm the capacity of drainage systems and cause flooding. In urban underpasses and rural canyons, this can result in deadly flash flooding.
Let’s Do Some Math And See How A Flash Flood Works
Let’s use an idealized system to illustrate the difference between a normal rain situation and a flash flood. Consider a basin the size of a bathtub with dimensions of five feet long, two feet wide, and two feet deep. The volume of this tub is twenty cubic feet.
There is a drain at the bottom of the bathtub. When the drain is open, water can flow out at a rate of 80 cubic feet per hour (I just did a little calculation in my head to get this number). When I turn on the water at half its capacity, water flows into the tub at a rate of 50 cubic feet per hour (I made it up — it’s my idealized tub). If the drain is open, all the water will go down the drain. Now suppose I turn on the water full-force, 100 cubic feet per hour. Now the drain cannot handle all the incoming water. With 100 coming in and 80 going out, my tub, which holds 20, will fill in an hour.
After that, the water will overflow into my living room — a flood.
Now suppose that the drain gets clogged with hair. If the tap is turned on full, the water could fill the tub in 12 minutes. If I leave the tap on for an hour, five tubs worth of water will be in my living room — a bigger flood.
Now think what would happen if I constructed a giant funnel above my bathtub with a diameter at the top of 100 feet. The area of this funnel is about 8,000 square feet. If one inch of rain fell on the funnel, about 700 cubic feet of water would accumulate. That would be 35 bathtubs full draining into my tub. If the inch of rain was distributed evenly over ten days, which is about the average in the US, three and a half tubs of water would accumulate each day — this would flood my living room if the drain was closed or clogged, but probably drain away out the door.
But suppose the rain fell in an hour and all drained right into the bathtub. Thirty-five tubs of water would inundate my living room rapidly — a flash flood.
How Does The Situation In Utah Relate To My Tub?
Hildale, Utah is in a narrow canyon with steep walls of rock. The bottom of the canyon is like my bathtub and the open top is like my funnel. When enough rain fell at the top of the funnel, the canyon suffered a flash flood.
In the case of Hildale, the water didn’t even have to reach the bottom of the canyon to cause flooding. So much water accumulated on the hillsides that it created a flood on the road above the lowest point. The fatalities were the result of cars on that road being swept away by the torrent.
In nearby Zion National Park, some rappellers were also caught in a flash flood when they were overtaken by a wall of water at the narrowest part of a river. Three died and several are still missing. The amount of rainfall that caused this flash flood was estimated at only a half inch.
Flash Flood: Heavy Rain In A Short Time
Thunderstorms can produce rainfall rates of over two inches per hour. In many places, less than an hour of rain of this intensity will create a flash flood. Slow-moving thunderstorms or ‘training’ storms that follow the same path, and which can produce up to ten inches of rain in several hours, are the most dangerous.
The National Weather Service issues flash flood watches and warnings – always pay attention to these warnings. It could save your life.
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